High current- carrying-capacity dual element fuse

ABSTRACT

A dual element fuse intended to carry large currents is provided with a plurality of fuse links for interrupting major fault currents, a spring biased solder joint overload current interrupting switching device being interposed in each of said plurality of fuse links. The overload current interrupting devices are arranged in two spaced planes parallel to the plane defined by the two blade contacts of the fuse. Each of the overload current switching devices includes a fixed terminal and movable terminal both enclosed in a sub-casing of electric insulating material, separating the aforementioned terminals from a body of pulverulent arc-quenching filler. The terminals are arranged in a pattern tending to maximize the spacing between furlgurites resulting from backburn of fuse links beyond the subcasings incident to interruption of overload currents the interruption of which is relatively onerous.

United States Patent m 1March 20, 1973 Kozacka [5 HIGH CURRENT- CARRYING- CAPACITY DUAL ELEMENT FUSE [75] Inventor: Frederick J. Kozacka, South Hampton, N.H.

[73] Assignee: The Chase-Shawmut Company,

Newburyport, Mass.

[22] Filed:- July 7, 1971 [21] Appl. No: 160,379

[52] US. Cl ..337/l64, 337/293 [51 Int. Cl. ..H0lh 85/04 [58] Field of Search ..337/161, 162, 164, 165, 166, 337/293, 163

[56] References Cited UNITED STATES PATENTS 3,483,501 12/1969 Kozocka ..337/l66 3,189,712 6/1965 Kozocka ..337/l6l 2,670,418 2/1954 Kozocka t ..337/l6l 3,294,937 12/1966 Kozocka ..337/l66 Primary Examiner-George Harris Assistant ExaminerF. E. Bell Atz0rneyErwin Salzer [5 7 ABSTRACT A dual element fuse intended to carry large currents is provided with a plurality of fuse links for interrupting major fault currents, a spring biased solder joint overload current interrupting switching device being interposed in each of said plurality of fuse links. The overload current interrupting devices are arranged in two' 7 Claims, 3 Drawing Figures PATENTEDHARZOIQN 122128222113 F edeiilbi JKozaaM VMMA W/m HIGH CURRENT- CARRYING-CAPACITY DUAL ELEMENT FUSE BACKGROUND OF THE INVENTION filler. The overload interrupting device effects are initiation preferably in air. To this end the overload interrupting device, or overload interrupting switch, may be arranged in an additional casing, or sub-casing, which is provided inside the casing of the fuse and separates on interruption of low overload currents the point of arc inception from the pulverulent arc-quenching filler which embeds the major fault current interrupting fuse link, or fuse links. It has been found to be desirable to interrupt small overload currents at two parallel connected breaks, and to arrange each of a pair of overload interrupting switches which are connected in parallel in a separate sub-casing. Such sub-casings are arranged within the main casing of the fuse, thus separating each of the two parallel-connected breaks from the pulverulent arc-quenching filler inside of the casing of the fuse and embedding the major fault current-interrupting fuse link means. U.S. Pat. No. 3,483,501 to Frederick J. Kozacka, issued Dec. 9, 1969 for ELECTRIC CARTRIDGE FUSE discloses such a dual overload interrupting switch and dual sub-casing fuse.

The limit current is the largest current which a fuse may carry for an indefinite period of time, the minimum fusing current is the smallest current required to form a circuit interrupting break, and the minimum interrupting current is the smallest current which a fuse is capable of interrupting satisfactorily. The minimum interrupting current may be much larger than the limit current, or the minimum fusing current. In other words, a small overload current may be sufficiently high to cause formation of a break, but the fuse may not be capable of interrupting the small overload current which caused the formation of the break. This may lead to extremely hazardous situations. A fuse may be capable of effectively interrupting a given small overload current in one test circuit, and the same fuse may fail in another test circuit imposing more onerous interrupting conditions upon the fuse. Standardization of test circuits including a standardization of the topology thereof is one of the means adopted to avoid fuse failures at interruption of extremely low overload currents. Another approach to the problem is to design dual element fuses for relatively high current ratings in such a way as to enable the fuses to effectively interrupt all overload currents, however small they may be, irrespective of the severity of the interruptingconditions in the circuit into which the fuse has been inserted.

I have found that the low overload current interrupting ability of a fuse according to the above referred-to U.S. Pat. No. 3,483,501 can be greatly increased, i.e., its minimum interrupting current reduced by increasing the number of overload interrupting switches which are connected in parallel, and by increasing correspondingly the number of'sub-casings so that each overload interrupting switch is enclosed in its individual sub-casing. This raises, however, the problem of how to arrange a large number of overload interrupting switches and of sub-casings within the limited space of a fuse casing of standard size, or less than standard size.

The sub-casings are preferably made of tubes ofinsulating material which are squashed and flattened at one side thereof, thus minimizing cost and space requirements, as taught in U.S. Pat. No. 3,483,501. In the structure of U.S. Pat. No. 3,483,501 the squashed and flattened ends of the two sub-casings are arranged closer to each other than the opposite ends of the subcasings whose cross-section is circular. It has been found necessary or desirable when increasing the number of overload current interrupting devices beyond two, and increasing the number of sub-casings beyond two, to reverse the positioning of the overload interrupting switches and of the sub-casings thereof as will be shown below more in detail.

SUMMARY OF THE INVENTION A fuse embodying this invention includes a tubular casing of electric insulating material, a body of pulverulent arc-quenching filler inside of said casing, and a pair.

of terminal caps each closing one of the ends of said casing, and a pair of blade contacts arranged in a common plane each projecting transversely through one end surface of one of said pair of terminal caps. A fuse embodying this invention further includes a plurality of perforated ribbon fuse links arranged inside said casing, submersed in said body of arc-quenching filler, each conductively interconnecting the axially inner ends of said pair of blade contacts. The fuse further includes a plurality of solder joint overload current interrupting switching devices each interposed into the current path of one of said plurality of ribbon fuse links. The aforementioned plurality of switching devices forms two groups of switching devices, i.e., a first group of switching devices and a second group of switching devices. The constituent switching devices of the first group are arranged in a first plane substantially parallel to said common plane of said pair of blade contacts, and the constituent switching devices of said second group are arranged in a second plane spaced from said first plane substantially parallel to said common plane of said pair of blade contacts. The fuse further includes a plurality of sub-casings equal in number to the number of said switching devices, each housing one of said plurality of switching devices, and separating each of said plurality of switching devices from said body of arc-quenching filler. The aforementioned plurality of sub-casings forms two groups of sub-casings, i.e., a first group of sub-casings and a second group of sub-casings. The constituent sub-casings of said first group are arranged in a first plane substantially parallel to said common plane of said pair of blade contacts and the constituent sub-casings of said second group are arranged in a second plane spaced from said first plane substantially parallel to said common plane of said pair of blade contacts.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a fuse embodying this invention and is substantially a section along l-l of FIG. 2;

FIG. 2 shows the same structure as FIG. 1 and is substantially a section along 2-2 of FIG. 1; and

FIG. 3 is substantially a longitudinal section of a single overload current interrupting device and the individual sublcasing thereof taken along 3-3 of FIG. 1 and drawn on a larger scale than FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENT In the drawings reference numeral 1 has been applied to indicate a tubular casing of electric insulating material, e.g., vulcanized fiber, or melamine-glasscloth. A body 2 of pulverulent arc-quenching filler is arranged inside of casing l. The pulverulent arcquenching filler may be quartz sand. A pair of terminal caps 3 closes the ends of casing 1. Reference numeral 4 has been applied to indicate a pair of blade contacts. Blade contacts 4 are arranged in a common plane and each projects transversely through one end surface of one of the two terminal caps 3. A plurality of perforated fuse links are arranged inside of easing l and submersed in arc-quenching tiller 2. Each fuse link includes two sections 5,6 of equal length. The axially outer ends of fuse link sections 5,6 are conductively connected to the axially inner ends of blade contacts 4. The axially inner end of fuse link sections 5,6 form a gap therebetween, as clearly shown in FIG. 3. This gap is normally bridged by a spring biased solder joint overload current interrupting switching device. The fuse link structure illustrated includes eight fuse links of which each is made up of a pair of fuse link sections 5,6, forming eight gaps normally bridged by eight overload current interrupting switching devices, one such device bridging each of the eight gaps. Each overload current interrupting switching device includes a stationary cylindrical terminal 7 and a movable spring biased terminal 8. Terminal 8 is provided with a collar 9 engaged by a helical spring 10. Spring housing 11 has a flange 12' engaged by the right end of spring 10 (FIG. 3), thus biasing terminal 8 from right to left. Terminal 7 closes one end of cylindrical spring housing 11 and the end surface of terminal 7 remote from terminal 8 is connected by a solder joint 12 to fuse link section 5. Solder joint 12 is made of a solder having a relatively high fusing point and is not supposed to fuse at any time. The movable terminal 8 has an end surface adjacent to fuse link section 6 which is provided with a groove into which the axially inner end of fuse link section 6 projects. A solder joint 13 made of a solder having a relatively low fusing point conductively connects movable terminal 8 to the axially inner end of fuse link section 6. A solderjoint 14 of a solder having a relatively low fusing point conductively connects movable terminal 8 to spring housing 11. Overload currents of inadmissible duration cause fusing of solder joints l3 and I4. As a result, movable terminal 8 is propelled from right to left (FIG. 3) under the bias of spring 10, thus causing formation of a break between movable terminal 8 and the axially inner end of fuse link section 6. The fixed terminal 7 has a larger diameter than terminal 8 and forms a cavity into which movable terminal 8 may enter when being moved under the bias of helical spring 10. Reference numeral 15 has been applied to indicate a sub-casing of insulating material. As shown in FIG. 3, the left cylindrical end of sub-casing 15 is mounted on spring housing 11 and the right end of subcasing 15 is squashed and flattened, and affixed by means of a staple 16 to fuse link section 6. The body 2 of pulverulent arc-quenching filler surrounds sub-casing 15, but has no access to the break initially formed between movable terminal 8 and the axially inner end of fuse link section 6. It is important that arc-initiation 5 at the junction between parts 6 and 8 occurs in a gaseous atmosphere rather than inside of a pulverulent arcquenching fulgurite-forming medium.

Parts 7, 8, 9, l and 11 are arranged in coaxial relation. The fuse link sections and 6 are of equal length, and are identical. All of the perforations, or points of reduced cross-sectional area, or necks, of fuse link sections 5 are embedded in the body 2 of arc-quenching filler, but not all of the perforations, or points of reduced cross-sectional area, or necks of fuse link sections 6 are exposed to the surrounding pulverulent arcquenching filler. One of the points of reduced crosssectional area of fuse link sections 6 is enclosed in subcasing forming a heat generator and heat dam to establish the right yielding point characteristic for solder joints I3 and 14.

Under severe interrupting conditions the arcs formed between movable terminals 8 and the axially inner ends of fuse link sections 6 will burn back to such an extent as to burn beyond the flattened ends of sub-casings 15 into the portion of fuse link sections 6 which are embedded in the arc-quenching filler 2. This may result in the formation of fulgurites if the arc-quenching filler is fulgurite-forming, e.g., consists of quartz sand. Under such circumstances it is important to maximize the spacing and to avoid merger of the fulgurites formed by the back-burn of fuse link sections 6 beyond the flattened ends of sub-casings 15, or beyond the end of subcasings 15 immediately adjacent to the movable terminals 8 of the overload switching devices.

As is apparent from FIGS. 1 and 2 four overload switching devices having terminals 7,8 as shown in FIG. 3 are arranged in an upper plane, and four overload switching devices having terminals 7,8 as shown in FIG. 3 are arranged in a lower plane, the upper and lower plane being parallel to the common plane of blade contacts 4. This two-plane-feature makes it possible to arrange a large number of overload interrupting switches in the relatively limited space available in a fuse casing of standard size, or even smaller than standard size. All sub-casings 15 are arranged in parallel relation to casing 1.

The fixed terminals 7 and the movable terminals 8 of contiguous switching devices in the upper plane and the fixed terminals 7 and the movable terminals 8. of contiguous switching devices in the lower plane are arranged in axially reverse positions. FIGS. 1 and 2 do not show the fixed terminals 7 and the movable terminals 8 of the overload switching devices, but the above geometry is apparent from FIGS. 1 and 2 since these figures show the non-squashed ends and the squashed ends of sub-casings l5, and since the fixed terminals 7 are always arranged adjacent a nonsquashed end of a sub-casing l5, and the movable terminal 8 of any switching device is always arranged adjacent the squashed and flattened end of a sub-casing FIG. 2 shows four subvcasings 15 The upper two sub-casings 15 house switching devices arranged in the upper plane and the lower two sub-casings 15 house switching devices arranged in the lower plane. The

squashed and flattened ends of superimposed sub-casings of switching devices of which one pertains to the upper layer of switching devices and the other pertains to the lower layer of switching devices are arranged in axially reverse positions, i.e., the squashed ends of superimposed sub-casing situated in the upper plane and the lower plane point to different blade contacts 4. These ends are spaced from each other a larger distance than the non-squashed ends of sub-casings 15, thus differing from the arrangement shown in the above referred-to US. Pat. No. 3,483,501. FIG. 2 does not show the relative positions of fixed and movable terminals 7,8 inside of sub-casings 15, but the relative position thereof is evident from FIG. 2 since each fixed terminal 7 is arranged immediately adjacent a nosquashed end of a sub-casing l5, and since each movable terminal 8 is arranged immediately adjacent a squashed and flattened end ofa sub-casing 15.

The above arrangement of terminals 7,8 in two different spaced layers and of casings 15 in two different spaced layers and the axially reverse arrangement of terminals 7,8 and sub-casings 15 maximizes the number of switching devices which can be provided in a given limited space and at the same time maximizes the spacing between fulgurites resulting from the back-burn of fuse link sections 6 beyond the confines of sub-casings The squashed flattened type of sub-casings minimizes the cost of production. In some instances it may be necessary, or desirable, to provide sub-casings which are circular in cross-section from one end to the other end thereof and are closed at the ends adjacent the movable terminals by insulating discs having slots for the passage of the fusible element sections 6 to which the movable terminals 8 are connected by solder joints 13. Such a structure should preferably be constructed in accordance with the teachings in the copending patent application of Richard A. Belcher, filed May l3, 1972 Ser. No. 142,877 for Electric Cartridge Fuse.

The axially reverse position scheme of the fixed terminals 7 and of the movable terminals 8 as set forth above should be adopted in the interest of fast recovery of dielectric strength irrespective of whether or not the sub-casings 15 are of the squashed flattened end type.

It will be apparent from FIG. 1 that the axial spacing between the squashed ends of contiguous pairs of subcasings 15 arranged in the same layer or same plane exceeds the axial spacing between the non-squashed ends of contiguous pairs of sub-casings 15 arranged in the same layer or plane. The same applies as to the axial spacing between squashed ends and the axial spacing between non-squashed ends of contiguous pairs of subcasings 15 which are superimposed, or arranged in different planes or layers. This is clearly apparent from FIG. 2. It is evident from FIG. 3 combined with FIG. 1 that the axial spacing between the axially outer ends of the movable terminals or movable members 8 of contiguous switching devices arranged in the same plane exceeds the axial spacing between the axially inner ends of the fixed terminals or fixed members 7 of contiguous pairs of switching devices arranged in the same plane. The same applies as is evident from FIG. 3 combined with FIG. 2 as to the axial spacing between the axially outer ends of the movable terminals or movable members 8 of contiguous pairs of switching devices arranged in different layers or planes and the axial spacing between the axially inner ends of the fixed terminals 7 of contiguous pairs of switching devices arranged in different layers or planes.

I claim as my invention:

1. An electric dual element high-current-carryingcapacity cartridge fuse including a. a tubular casing of electric insulating material;

b. a body of pulverulent arc-quenching filler inside of said casing;

c. a pair of terminal caps each closing one of the ends of said casing;

d. a pair of blade contacts arranged in a common plane each projecting transversely through one end surface of one of said pair of terminal caps;

e. a plurality of perforated ribbon fuse links inside said casing submersed in said body of arequenching filler each conductively interconnecting the axially inner ends of said pair of blade contact;

f. a plurality of spring. biased solder joint overload current interrupting switching devices each interposed into the current path of one of said plurality of ribbon fuse links, said plurality of switching devices forming a first group and a second group of switching devices, the constituent switching devices of said first group being arranged in a first plane substantially parallel to said common plane of said pair of blade contacts and the constituent switching devices of said second group being arranged in a second plane spaced from said first plane substantially parallel to said common plane of said pair of blade contacts; and j a plurality of sub-casings of electric insulating material inside of said casing equal in number to the number of said switching devices each housing one of said plurality of switching devices and each separating one of said plurality of switching devices from said body of arc-quenching filler, said plurality of sub-casings forming a first group and a second group of sub-casings, the constituent subcasings of said first group being arranged in a first plane substantially parallel to said common plane of said pair of blade contacts and the constituent sub-casings of said second group being arranged in a second plane spaced from said first plane substantially parallel to said common plane of said pair of blade contacts.

. An electric fuse as specified in claim 1 wherein a. each of said plurality of switching devices includes a fixed terminal and a spring biased movable terminal;

. said fixed terminal and said movable terminal of contiguous pairs of switching devices of said first group are arranged in axially reverse positions in such a way that the axial spacing between the axially outer ends of the movable terminals of contiguous pairs of switching devices exceeds the axial spacing between the axially inner ends of the fixed terminals of contiguous pairs of switching devices; and wherein c. said fixed terminal and said movable terminal of contiguous switching devices of said second group are arranged in axially reverse positions in such a way that the axial spacing between the axially outer ends of the movable terminals of contiguous pairs of switching devices exceeds the axial spacing between the axially inner ends of the fixed terminals of contiguous pairs of switching devices.

3. An electric fuse as specified in claim 1 wherein a. each of said plurality of switching devices includes a fixed terminal and a movable spring biased terminal; and wherein b. pairs of superimposed switching devices of which one pertains to said first and the other pertains to said second group are arranged in axially reverse positions in such a way that the axial spacing between the axially outer ends of the movable terminals of contiguous pairs of switching devices exceeds the axial spacing between the axially inner ends of the fixed terminals of contiguous pairs of switching devices.

4. An electric fuse as specified in claim 1 wherein a. each of said plurality of switching devices includes a fixed terminal and a spring biased movable terminal;

b. said fixed terminal and said movable terminal of contiguous switching devices of said first group are arranged in axially reverse positions in such a way that the axial spacing between the axially outer ends of the movable terminals of contiguous pairs of switching devices exceeds the axial spacing between the axially inner ends of the fixed terminals of contiguous pairs of switching devices;

c. said fixed terminal and said movable terminal of the contiguous switching devices of said second group are arranged in axially reverse positions in such a way that the axial spacing between the axially outer ends of the movable terminals of contiguous pairs of switching devices exceeds the axial spacing between the axially inner ends of the fixed terminals of contiguous pairs of switching devices; and wherein (1. pairs of superimposed switching devices of which one pertains to said first group and the other pertains to said second group of switching devices are arranged in axially reverse positions in such a way that the axial spacing between the axially outer ends of the movable terminals of contiguous pairs of switching devices exceeds the axial spacing between the axially inner ends of the fixed terminals of contiguous pairs of switching devices.

. An electric dual element cartridge fuse including a tubular casing of insulating material;

. a body of pulverulent arc-quenching filler inside of said casing;

c. a pair of terminal caps each closing one of the ends of said casing;

d. a pair of blade contacts arranged in a common plane each projecting transversely through the end surface of one of said pair of terminal caps;

e. a plurality of ribbon fuse links having serially related points of reduced cross-sectional area, each of said plurality of fuse links including a pair of sections of equal length having axially outer ends conductively connected to the axially inner ends of said pair of blade contacts and axially inner ends forming a gap therebetween;

f. a plurality of overload current interrupting switching devices each normally conductively interconnecting said axially inner ends of a pair of sections of said plurality of fuse links, each of said plurality of switching devices including a fixed cylindrical terminal of relatively large diameter having an. end surface conductively connected to the axially inner end of one of said sections of one of said plurality of fuse links, each of said plurality of switching devices further including a movable spring biased terminal arranged in coaxial relation to said fixed terminal thereof and having an'end surface conductively connected by a solderjoint to the axially inner end of one of said sections of one of said plurality of fuse links, said plurality of switching devices forming a first group and a second group of switching devices, the constituent switching devices of said first group being arranged in a first plane substantially parallel to said common plane of said pair of blade contacts and the constituent switching devices of said second group being arranged in a second plane spaced from said first plane and substantially parallel to said common plane of said pair of blade contacts;

g. a plurality of substantially tubular sub-casings of electric insulating material arranged inside of said casing in parallel relation to said casing, each of said plurality of sub-casings housing one of said plurality of switching devices and each of said plurality of sub-casings having an end circular in cross-section arranged immediately adjacent said fixed terminal of one of said plurality of switching devices and each of said plurality of switching devices having a squashed and flattened end arranged immediately adjacent said movable terminal of one of said plurality of switching devices;

h. said squashed and flattened ends of said sub-casings of contiguous switching devices of said first group of switching devices being arranged in axially reverse positions in such a way that the axial spacing between the squashed and flattened ends of contiguous pairs of switching devices exceeds the axial spacing between the ends circular in cross-section of contiguous pairs of sub-casings;

i. said squashed and flattened ends of said sub-casings of contiguous switching devices of said second group of switching devices being arranged in axially reverse positions in such a way that the axial spacing between the squashed and flattened ends of contiguous pairs of switching devices exceeds the axial spacing between the ends circular in cross-section of contiguous pairs of sub-casings; and

j. the squashed and flattened ends of said sub-casings of superimposed switching devices of which one pertains to said first group of switching devices and the other pertains to said second group of switching devices being arranged in axially reverse positions in such a way that the spacing between the squashed and flattened ends of the sub-casings of superimposed switching devices exceeds the spacing between the ends circular in cross-section of the sub-casings of superimposed switching devices.

6. An electric fuse including a. a tubular casing of electric insulating material;

a body of pulverulent arc-quenching filler inside said casing;

a pair of terminal caps each closing one of the ends of said casing;

a pair of blade contacts arranged in a common plane each projecting transversely through one end surface of one of said pair of terminal caps;

. a plurality of perforated ribbon fuse links inside said casing submersed in said arc-quenching filler, said plurality of fuse links forming two separate groups of fuse links each arranged in one of a pair of parallel planes parallel to said common plane of 'said pair of blade contacts, said plurality of fuse links having axially outer ends connected to said pair of blade contacts and having axially inner spaced ends;

. a plurality of spring-biased solder joint overload current switching devices each arranged in one of said pair of parallel planes and each normally conductively interconnecting said axially inner spaced ends of said plurality of ribbon fuse links, each of said plurality of switching devices including a fixed non-spring-biased member and a movable springbiased member;

. a plurality of sub-casings each housing one of said plurality of switching devices and separating each of said plurality of switching devices from said arequenching filler; and

. said switching devices being arranged in reverse d. a pair of blade contacts arranged in a common plane each projecting transversely through one end surface of one of said pair ofterminal caps;

. a plurality of perforated ribbon fuse links inside said casing submersed in said arc-quenching filler, said plurality of fuse links forming two separate groups of fuse links each arranged in one of a pair of parallel planes parallel to said common plane of said pair of blade contacts, said plurality of fuse links having axially outer ends connected to said pair of blade contacts and having axially inner spaced ends;

. a plurality of spring biased solder joint overload current interrupting switching devices each arranged in one of said pair of parallel planes and each normally conductively interconnecting said axially inner spaced ends of said plurality of ribbon fuse links, each of said plurality of switching devices including a first contact fixed relative to said plurality of fuse links and a second contact movable relative to said plurality of fuse links;

. a plurality of sub-casings of electric insulating material inside said casing equal in number to the number of said switching devices each housing one of said plurality of switching devices and each separating one of devices from said body of arc-quenching filler, each of said plurality of sub-casings having one end circular in cross-section arranged immediately adjacent said first contact of one of said plurality of switching devices and each of said plurality of sub-casings having another end being squashed and flattened arranged immediately adjacent said second contact of one of said plurality of switching devices; and

. contiguous switching devices and said sub-casings thereof being arranged in reverse in such a way that the spacing between said squashed and flattened ends of contiguous said sub-casings exceeds the spacing between said end circular in cross-section of contiguous said sub-casings.

said plurality of switching- 

1. An electric dual element high-current-carrying-capacity cartridge fuse including a. a tubular casing of electric insulating material; b. a body of pulverulent arc-quenching filler inside of said casing; c. a pair of terminal caps each closing one of the ends of said casing; d. a pair of blade contacts arranged in a common plane each projecting transversely through one end surface of one of said pair of terminal caps; e. a plurality of perforated ribbon fuse links inside said casing submersed in said body of arc-quenching filler each conductively interconnecting the axially inner ends of said pair of blade contact; f. a plurality of spring biased solder joint overload current interrupting switching devices each interposed into the current path of one of said plurality of ribbon fuse links, said plurality of switching devices forming a first group and a second group of switching devices, the constituent switching devices of said first group being arranged in a first plane substantially parallel to said common plane of said pair of blade contacts and the constituent switching devices of said second group being arranged in a second plane spaced from said first plane substantially parallel to said common plane of said pair of blade contacts; and g. a plurality of sub-casings of electric insulating material inside of said casing equal in number to the number of said switching devices each housing one of said plurality of switching devices and each separating one of said plurality of switching devices from said body of arc-quenching filler, said plurality of sub-casings forming a first group and a second group of sub-casings, the constituent sub-casings of said first group being arranged in a first plane substantially parallel to said common plane of said pair of blade contacts and the constituent sub-casings of said second group being arranged in a second plane spaced from said first plane substantially parallel to said common plane of said pair of blade contacts.
 2. An electric fuse as specified in claim 1 wherein a. each of said plurality of switching devices includes a fixed terminal and a spring biased movable terminal; b. said fixed terminal and said movable terminal of contiguous pairs of switching devices of said first group are arranged in axially reverse positions in such a way that the axial spacing between the axially outer ends of the movable terminals of contiguous pairs of switching devices exceeds the axial spacing between the axially inner ends of the fixed terminals of contiguous pairs of switching devices; and wherein c. said fixed terminal and said movable terminal of contiguous switching devices of said second group are arranged in axially reverse positions in such a way that the axial spacing between the axially outer ends of the movable terminals of contiguous pairs of switching devices exceeds the axial spacing between the axially inner ends of the fixed terminals of contiguous pairs of switching devices.
 3. An electric fuse as specified in claim 1 wherein a. each of said plurality of switching devices includes a fixed terminal and a movable spring biased terminal; and wherein b. pairs of superimposed switching devices of which one pertains to said first and the other pertains to said second group are arranged in axially reverse positions in such a way that the axial spacing between the axially outer ends of the movable terminals of contiguous pairs of switching devices exceeds the axial spacing between the axially inner ends of the fixed terminals of contiguous pairs of switching devices.
 4. An electric fuse as specified in claim 1 wherein a. each of said plurality of switching devices includes a fixed terminal and a spring biased movable terminal; b. said fixed terminal and said movable terminal of contiguous switching devices of said first group are arranged in axially reverse positions in such a way that the axial spacing between the axially outer ends of the movable terminals of contiguous pairs of switching devices exceeds the axial spacing between the axially inner ends of the fixed terminals of contiguous pairs of switching devices; c. said fixed terminal and said movable terminal of the contiguous switching devices of said second group are arranged in axially reverse positions in such a way that the axial spacing between the axially outer ends of the movable terminals of contiguous pairs of switching devices exceeds the axial spacing between the axially inner ends of the fixed terminals of contiguous pairs of switching devices; and wherein d. pairs of superimposed switching devices of which one pertains to said first group and the other pertains to said second group of switching devices are arranged in axially reverse positions in such a way that the axial spacing between the axially outer ends of the movable terminals of contiguous pairs of switching devices exceeds the axial spacing between the axially inner ends of the fixed terminals of contiguous pairs of switching devices.
 5. An electric dual element cartridge fuse including a. a tubular casing of insulating material; b. a body of pulverulent arc-quenching filler inside of said casing; c. a pair of terminal caps each closing one of the ends of said casing; d. a pair of blade contacts arranged in a common plane each projecting transversely through the end surface of one of said pair of terminal caps; e. a plurality of ribbon fuse links having serially related points of reduced cross-sectional area, each of said plurality of fuse links including a pair of sections of equal length having axially outer ends conductively connected to the axially inner ends of said pair of blade contacts and axially inner ends forming a gap therebetween; f. a plurality of overload current interrupting switching devices each normally conductively interconnecting said axially inner ends of a pair of sections of said plurality of fuse links, each of said plurality of switching devices including a fixed cylindrical terminal of relatively large diameter having an end surface conductively connected to the axially inner end of one of said sections of one of said plurality of fuse links, each of said plurality of switching devices further including a movable spring biased terminal arranged in coaxial relation to said fixed terminal thereof and having an end surface conductively connected by a solder joint to the axially inner end of one of said sections of one of said plurality of fuse links, said plurality of switching devices forming a first group and a second group of switching devices, the constituent switching devices of said first group being arranged in a first plane substantially parallel to said common plane of said pair of blade contacts and the constituent switching devices of said second group being arranged in a second plane spaced from said first plane and substantially parallel to said common plane of said pair of blade contacts; g. a plurality of substantially tubular sub-casings of electric insulating material arranged inside of said casing in parallel relation to said casing, each of said plurality of sub-casings housing one of said plurality of switching devices and each of said plurality of sub-casings having an end circular in cross-section arranged immediately adjacent said fixed terminal of one of said plurality of switching devices and each of said plurality of switching devices having a squashed and flattened end arranged immediately adjacent said movable terminal of one of said plurality of switching devices; h. said squashed and flattened ends of said sub-casings of contiguous switching devices of said first group of switching devices being arranged in axially reverse positions in such a way that the axial spacing between the squashed and flattened ends of contiguous pairs of switching devices exceeds the axial spacing between the ends circular in cross-section of contiguous pairs of sub-casings; i. said squashed and flattened ends of said sub-casings of contiguous switching devices of said second group of switching devices being arranged in axially reverse positions in such a way that the axial spacing between the squashed and flattened ends of contiguous pairs of switching devices exceeds the axial spacing between the ends circular in cross-section of contiguous pairs of sub-casings; and j. the squashed and flattened ends of said sub-casings of superimposed switching devices of which one pertains to said first group of switching devices and the other pertains to said second group of switching devices being arranged in axially reverse positions in such a way that the spacing between the squashed and flattened ends of the sub-casings of superimposed switching devices exceeds the spacing between the ends circular in cross-section of the sub-casings of superimposed switching devices.
 6. An electric fuse including a. a tubular casing of electric insulating material; b. a body of pulverulent arc-quenching filler inside said casing; c. a pair of terminal caps each closing one of the ends of said casing; d. a pair of blade contacts arranged in a common plane each projecting transversely through one end surface of one of said pair of terminal caps; e. a plurality of perforated ribbon fuse links inside said casing submersed in said arc-quenching filler, said plurality of fuse links forming two separate groups of fuse links each arrangEd in one of a pair of parallel planes parallel to said common plane of said pair of blade contacts, said plurality of fuse links having axially outer ends connected to said pair of blade contacts and having axially inner spaced ends; f. a plurality of spring-biased solder joint overload current switching devices each arranged in one of said pair of parallel planes and each normally conductively interconnecting said axially inner spaced ends of said plurality of ribbon fuse links, each of said plurality of switching devices including a fixed non-spring-biased member and a movable spring-biased member; g. a plurality of sub-casings each housing one of said plurality of switching devices and separating each of said plurality of switching devices from said arc-quenching filler; and h. said switching devices being arranged in reverse and in such a way that the axial spacing between the axially outer ends of movable spring-biased members of contiguous pairs of switching devices exceeds the axial spacing between the axially inner ends of fixed non-spring-biased members of contiguous pairs of switching devices.
 7. An electric cartridge fuse including a. a tubular casing of electric insulating material; b. a body of pulverulent arc-quenching filler inside said casing; c. a pair of terminal caps each closing one of the ends of said casing; d. a pair of blade contacts arranged in a common plane each projecting transversely through one end surface of one of said pair of terminal caps; e. a plurality of perforated ribbon fuse links inside said casing submersed in said arc-quenching filler, said plurality of fuse links forming two separate groups of fuse links each arranged in one of a pair of parallel planes parallel to said common plane of said pair of blade contacts, said plurality of fuse links having axially outer ends connected to said pair of blade contacts and having axially inner spaced ends; f. a plurality of spring biased solder joint overload current interrupting switching devices each arranged in one of said pair of parallel planes and each normally conductively interconnecting said axially inner spaced ends of said plurality of ribbon fuse links, each of said plurality of switching devices including a first contact fixed relative to said plurality of fuse links and a second contact movable relative to said plurality of fuse links; g. a plurality of sub-casings of electric insulating material inside said casing equal in number to the number of said switching devices each housing one of said plurality of switching devices and each separating one of said plurality of switching devices from said body of arc-quenching filler, each of said plurality of sub-casings having one end circular in cross-section arranged immediately adjacent said first contact of one of said plurality of switching devices and each of said plurality of sub-casings having another end being squashed and flattened arranged immediately adjacent said second contact of one of said plurality of switching devices; and h. contiguous switching devices and said sub-casings thereof being arranged in reverse in such a way that the spacing between said squashed and flattened ends of contiguous said sub-casings exceeds the spacing between said end circular in cross-section of contiguous said sub-casings. 